My U.S. Pat. No. 5,416,979, issued May 23, 1995, discloses apparatus for drying a wet paper web during manufacture of the paper web. The apparatus allows the moisture profile of the web to be carefully controlled and adjusted. The apparatus includes a rotatable yankee dryer drum and a hood partially encompassing the rotatable dryer drum. A plurality of elongated heater nozzle boxes are disposed in the hood interior and extend across the dryer drum in the cross-machine direction, the elongated heater nozzle boxes being arrayed side-by-side in the machine direction. A plurality of gas burners is located in each of the nozzle box interiors and arranged side-by-side along the length of the nozzle box interiors for producing hot combustion gases within the nozzle box interiors of the heater nozzle boxes.
My U.S. Pat. No. 5,425,852, issued Jun. 20, 1995, discloses a system for reducing blistering of a wet paper web on a yankee dryer. Blistering is controlled by directing compressed dry air toward the wet paper web in the space between a pressure roll external of the yankee dryer hood and the entry opening between the dryer hood and the yankee dryer drum. The flow of compressed air is controlled so that air flow can be modified across the full width of the wet paper web.
My U.S. Pat. No. 5,465,504, issued Nov. 14, 1995, discloses drying a paper web on a yankee dryer. The moisture profile is controlled by arraying a plurality of gas burners in a cross-machine direction within the yankee dryer hood to direct heat toward incremental width portions of the paper web.
The present invention relates to drying a wet web and modifying the moisture profile. In the disclosed embodiment, the invention is employed to modify the moisture profile of a wet paper web while the paper web is being manufactured in a system utilizing a through air dryer; however, the principles of the invention are applicable to drying other types of webs. Through air dryers are well known expedients for drying wet paper webs, particularly tissue webs, conventionally employing a through air dryer roll for supporting and transporting the wet paper web and a through air dryer hood cooperable with the through air dryer roll to cause the passage of hot air through the through air dryer roll and the wet paper web thereon to dry the wet paper web. Flat bed through air dryers not employing a through air dryer roll and instead employing other forms of web support structure are also known and the principles of the present invention are applicable to such systems as well.
Through air dryers are shown, for example, in the following United States patents: U.S. Pat. No. 3,303,576, issued Feb. 14, 1967, U.S. Pat. No. 3,432,936, issued Mar. 18, 1969, and U.S. Pat. No. 3,821,068, issued Jun. 28, 1974.
It is, of course, highly desirable to maintain a uniform moisture profile in a paper web during manufacture thereof in order to attain a high quality end product of uniform character. Through air dry systems present major problems with respect to obtaining and maintaining web uniform moisture profiles.
The problem is that through air dryers rely upon the passage of heated air through the wet paper web to accomplish drying. Any differences in the moisture profile of the wet paper web are actually amplified during the through drying process since web permeability is higher in the drier areas of the web and these areas will tend to dry faster than the wetter, less permeable portions of the web. In other words, the drier portions of the web tend to get even drier relative to the wetter portions. In order to overcome this problem it has been the practice in the prior art to "over dry" the entire web in order to obtain a degree of uniform dryness. This, of course, is highly wasteful of energy.
Conventional through air dryer arrangements have other drawbacks as well. Typically, conventional through air dryers require large volume combustion and air mixing chambers aimed at uniformly distributing heat. These arrangements take up considerable physical space and are capital intensive. Through air dryers conventionally employ structures within the hood interior, such as turning vanes, which are for the purpose of receiving heated air from a separate combustion chamber and distributing it uniformly prior to engagement with the wet paper web being dried in an attempt to maintain a uniform moisture profile. These mechanisms are not only complicated and expensive but are not as effective as desired from the stand point of maintaining a level moisture profile. Fan pressure loss and requirement for bulky and extensive duct work are additional problems in the prior art.
In addition to the patents noted, above the following United States patents are considered representative of the current state of the prior art: U.S. Pat. No. 5,397,437, issued Mar. 14, 1995, U.S. Pat. No. 5,174,046, issued Dec. 29, 1992, U.S. Pat. No. 5,150,535, issued Sep. 29, 1992, U.S. Pat. No. 4,654,981, issued Apr. 7, 1987, U.S. Pat. No. 4,462,868, issued Jul. 31, 1984, U.S. Pat. No. 3,919,783, issued Nov. 18, 1975, U.S. Pat. No. 3,891,500, issued Jun. 24, 1975, U.S. Pat. No. 3,541,697, issued Nov. 24, 1970, and U.S. Pat. No. 3,340,617, issued Sep. 12, 1967.